Transient, high oxidation state, metal-oxo species are key intermediates in life sustaining biological energy conversion transformations, such as water oxidation at the oxygen-evolving complex (OEC) in photosynthesis II, and cellular substrate oxidation at heme iron in cytochrome P450s, peroxidases, and catalases. Despite the opposite function of the two systems (O2 cleavage; P450, O2 formation; PS II), the principle design feature involves managing oxygen atom transfer (OAT) from highly reactive metal-oxo fragments. In each case, a unique ligand supports an otherwise unstable high oxidation state metal-oxo species. In the oxygen evolving complex of PS II, clustered multiple Mn centers of the S4 Kok state (Mn(IV)3Mn(V)) act as elaborate ligands for a manganyl-oxo intermediate. For P450, heme radical cation formation enables ferryl Fe(IV)=O to exist. Not all high oxidation state metal-oxo species are productive for living systems. For example, Cr(VI) is a carcinogen and causes cellular oxidative DNA damage. However, Cr(VI) is not an active oxidant, instead, ascorbic or glutathione reduction to Cr(III) leads to formation of Cr(IV) and Cr(V) intermediates.
Generating reactive intermediates incurs negative outcomes even nature cannot avoid. Oxidative enzyme degradation pathways are inevitable but protein repair processes are built-in. In artificial systems, the catalytic cycle ends once the ligand degrades, thus limiting turnover. Much of the challenge in developing OAT catalytic systems is to obtain robust, degradation impervious catalysts. To control the geometry and electronic properties of metal ions, chemists create customized ligands to mimic nature. Porphyrin, corrole, catechol, 2-hydroxyacid, and salen ligands can stabilize Cr(V), including a few structurally characterized examples. Some Cr(V)=O oxo complexes are known that participate as catalysts in OAT to sulfides, phosphines, olefins, alkynes, and enantioselective adaptations but are relatively unstable. A stable Cr complex would permit OAT in a commercially viable manner. Hence, there is a need for custom ligands, catalytic systems and their use in catalytic oxidation of organic compounds.